The present invention relates generally to the control of reheat furnaces used in association with metal rolling mills and more particularly to a method of controlling the use of the furnace for controlling the variations in deformation resistance of a workpiece to be rolled which variations are occasioned by the existence of support skids upon which the workpiece resides while within a portion of the furnace.
In the discipline of metal rolling, it is common practice to employ what is known as a reheat furnace to bring slabs or metal workpieces to the proper temperature for subsequent rolling. Such furnaces normally include at least one so-called heat zone and one soak zone. In the heat zone, heat is usually supplied to the workpiece, from burners located above and below the slab or workpiece which is pushed through the furnace while residing on a plurality of support skids. Quite often these skids take on the form of longitudinal pipes extending the length of the heat zone and having a hollow interior through which cooling fluid, such as water, is passed to prevent too rapid deterioration and to maintain the strength of the pipes. Insulating material such as a ceramic is placed upon the pipe such that the workpiece actually resides upon the the insulating material. The insulation is not, however, perfect. In addition, the skids shadow the workpiece from the burners such that temperature gradients (commonly called skid marks) exist within the metal workpiece. These gradients, if allowed to continue to exist at the time of rolling, result in differences in the deformation resistance of the material and increase the difficulty of rolling consistent gage metal.
Were the metal workpieces to be pushed through the furnace in the direction in which they are rolled, the skid marks would be along the length of the rolling path and would create no great problem. However, such a system would make the furnace inordinately long and it is, therefore, the customary practice to push slabs through the furnace such that they reside on the skids in a manner such that the skid marks are disposed transversely to the direction of rolling. As such, the skid marks represent a cyclic variation in material hardness during rolling which presents problems in maintaining consistent gage material.
Much has been written about skid marks and skid mark effects and much effort has gone into the design of the skids to minimize this effect. For more complete understanding of skid marks and their associated problems, reference is made to the following articles: "Influence of Skid Mark Design on Skid Mark Formation" by R. L. Howells, et al.; Journal of the Iron and Steel Institute, January, 1972; "Formation of Skid Marks in a Slab-Reheating Furnace" by F. M. Salter; The Iron and Steel Engineer's Group of the Iron and Steel Institute, Energy Management of Iron and Steel Works, Publication Number 105, London, England, April, 1967 (pages 151-174) and "Where Does the Energy Go? Design Basis vs. Average Monthly Operation" by James E. Hovis; Iron and Steel Engineer, December, 1978.
Automatic gage control is, of course, available in most hot rolling mills but if the skid marks are severe, such gage controls are often not adequate to compensate for the resulting variations in deformation resistance of the workpiece. Even where the gage control system is adequate to compensate for these variations, undesirable variations in workpiece shape and metalurgical quality may remain. Earlier mention was made of the soak zone and it is a primary purpose of the soak zone, in which the metal workpiece does not reside upon skids but sits on a continuous surface, to allow the skid mark effects to dissipate and for the slab to reach a more uniform temperature. It is, of course, one possible solution to leave the slabs in the soak zone for a sufficient length of time to insure that the skid marks will be completely dissipated under all expected operating conditions. This is not practical in most instances since this would reduce furnace efficiency and productivity and since there are practical economic limits to the design length of the soak zone. Neither is such completely necessary since some temperature gradient or skid mark effect is permissible so long as the effects are within acceptable limits.
The problems associated with skid marks are complicated by the fact that, as the insulation on the skids wears, the skid marks tend to be more pronounced. As pointed out in the Hovis article referenced above, the variation may be as much as four to one depending upon the condition of the insulation. It has been suggested to provide a model of the anticipated skid mark and to control the time in the soak zone as a function of this model. Since most modern hot rolling mills include some form of computer control, such modeling could be readily achieved. It is not practical to provide an accurate model for use over long periods of time, however, since it is not possible to accurately predict the manner in which the insulation will wear.
A further problem associated with the inability to accurately predict insulation wear is that of determining the appropriate time for replacing the skid insulation. At the present time, this determination is usually based upon the judgement of mill operating management without benefit of any quantitative evaluation of the skid mark effects on workpiece quality.